This invention relates to coffee bean roasting and more particularly to method and apparatus for the continuous roasting of coffee by microwave energy.
Roasting of coffee involves several steps, the first of which involves raising the temperature of the coffee bean to a level at which the residual internal moisture begins to be driven off. After having reached this level the coffee bean temperature rises more slowly until dry. Thereafter the bean elevates in temperature until it reaches approximately 350.degree.-375.degree. F. and roasting begins in which the beans undergo certain chemical and physical changes. More specifically, roasting involves the thermal decomposition of sugars such as sucrose, along with chemical interactions between the sugars, proteins and more complex carbohydrates to produce a variety or organic acids, aldehydes, ketones and the like which give roasted coffee its characteristic flavor and aroma. The coffee bean is also expanded during the final stages of roasting, and when the beans are ground after roasting, the essential contents are easily water extracted. During the period in which the coffee bean is raised in temperature to that which is necessary to evaporate the moisture and beyond to the temperature at which roasting takes place, the bean is endothermic. As roasting proceeds to the point where the sugars begin to decompose, the coffee bean becomes exothermic, and, if the beans become overheated they are subject to thermal runaway and burn, and may even flame. Without elaborating unduly, many coffee roasting procedures and associated apparatus have been proposed. One typical and common roaster, known as the Thermalo brand, manufactured by Jabez Burns Division of Blaw-Knox Food and Chemical Equipment, Inc. of Buffalo, New York, has been widely adopted, particularly in the United States. It is illustrative of present common commercial practice. One form of this apparatus is designed for continuous processing and is based upon the principle of burning a gas or fuel oil so as to create a high velocity stream of extremely hot gases having a temperature of the order of 800.degree. F. These gases are passed through a continuous roaster having a rotating perforated cylinder with an internal helical baffle which serves as a feed and advancng mechanism for the beans as the cylinder rotates about a horizontal axis. Hot gases passing through the cylinder wall perforations are sufficiently forceful and turbulent to stir through the beans and generally provide the convective heat transfer for the roasting process. Thus, this type of conventional roasting subjects the beans to high velocity hot air by the direct combustion with natural gas or fuel. When these gases leave the roaster after the last contact with the roasted beans, their temperature is typically 425.degree. F. often regarded as an approximate roastng temperature. By using this sysem, the hot air contacting the beans ultimately raises the surface temperature of the bean considerably higher than necessary for proper bean roast and causes greater thermal decomposition to take place at the surface of the bean than is necessary or desirable. This appears to be an unavoidable result of any system in which convective or conductive heating of the bean surface is relied upon to get a sufficient interior temperature to obtain a properly roasted bean throughout its cross-section. Thus, overroasting occurring at the surface of the bean in conventional systems and part way into the bean results in a greater weight loss than necessary compared to a bean heated uniformly to roasting temperature. In addition to the accompanying weight loss is also some loss of desirable flavor components and results in roasting process off-gases containing considerable amounts of decomposition products, smoke, tars and so forth, all of which represent major pollution control problems. Another causing lowering the yield in conventional roasting results from the use of a large excess of hot air conducted past the beans at high velocity (for efficiency). This results in removal of many fine and small particles of the roasted bean that could otherwise be a useful part of a conventional grind and contribute to the final product. However, these materials are usually non-recoverable, being taken up the exhaust stack and if recirculated back through the burner, they are consumed.
Several alternate proposals have been made to utilize different systems; however, they have not met with significant commercial success for several reasons. Reference is made to the book Coffee Processing Technology, Vol. I, by Michael Sivertz and H. Elliott Foote, published by A.V.I. Publishing, Inc. of Westport, Connecticut, 1963, at about pages 216 and following where several alternate systems are discussed. As previously mentioned, any system requiring external means for conduction or convection heating of the bean by a gas or liquid from the surface will inherently possess many of the foregoing disadvantages. Infrared and dielectric roasting of coffee is known but as reviewed in the referenced text, they have not been particularly successful. As noted on page 220 of the text, microwave heating for the purpose of roasting coffee has been proposed and is there reviewed on a laboratory experiment scale where it is found that the selectively roasted coffees form electric coronas of ionized gases. The conclusion drawn is that one cannot expect microwave energy to be useful for this purpose.
Microwave roasting has also been attempted to be used in conventional microwave ovens. However, the nonuniformity of fields within such ovens, even where movement of the beans is accomplished by rotating tray equipment, has also been unsuccessful due to lack of bean to bean contact enabling heat redistribution. In addition, this difficulty does not appear to be solvable in conventional continuous belt microwave equipment for similar reasons, namely, selective over roasting, burning and flaming of certain beans while others are under roasted. It should be noted that the foregoing problems result from the inherent character of coffee beans in that a statistical distribution of moisture content, oil content, sugar content, and other factors is sufficiently wide that it cannot be expected to achieve a uniform roast in which individual beans are subjected to substantially individualized treatment whether uniform or not. Nevertheless, it would be desirable to provide a microwave coffee bean roasting system since it is capable of inherently heating and roasting the bean uniformly across its cross-section and to avoid bean surface over roasting. There is, therefore, a general need for a microwave system for the roasting of coffee beans which will overcome the foregoing limitations and disadvantages of conventional roasting hot air systems, dielectric systems, and previous attempts at microwave roasting.